Geometric and Scaling Effects in the Speed of Catalytic Enzyme Micropumps

Abstract

Self-powered, biocompatible pumps in the nanometer to micron length scale have the potential to enable technology in several fields, including chemical analysis and medical diagnostics. Chemically powered, catalytic micropumps have been developed but are not able to function well in biocompatible environments due to their intolerance of salt solutions and the use of toxic fuels. In contrast, enzymatically powered catalytic pumps offer good biocompatibility, selectivity, and scalability, but their performance at length scales below a few millimeters, which is important to many of their possible applications, has not been well tested. Here, urease-based enzyme pumps of millimeter and micrometer dimensions were fabricated and studied. The scaling of the pumping velocity was measured experimentally and simulated by numerical modeling. Pumping speeds were analyzed accurately by eliminating Brownian noise from the data using enzyme patches between 5 mm and 350 μm in size. Pumping speeds of microns per second could be achieved with urease pumps and were fastest when the channel height exceeded the width of the catalytic pump patch. In all cases, pumping was weak when the dimensions of the patch were 100 μm or less. Experimental and simulation results were consistent with a density-driven pumping mechanism at all sizes studied and served as a framework for the in silico study of more complex two-dimensional (2D) and three-dimensional (3D) geometries. Attempts to create directional flow by juxtaposing inward and outward pumps were unsuccessful because of the symmetry of convection rolls produced by millimeter-size pump patches and the slow speeds of smaller pumps. However, simulations of a corrugated ratchet structure showed that directional pumping could be achieved with pump patches in the millimeter size range.

Programming Fluid Motion Using Multi-Enzyme Micropump Systems

“…where 𝑝𝑝, 𝐈𝐈, 𝜇𝜇, T, and ∇ are the respective pressure, unit tensor, 20,29 fluid dynamic viscosity, temperature, and spatial gradient operator, respectively. 𝜌𝜌 is the real-time fluid density and 𝐮𝐮 is the fluid velocity.…”

“…If the molecular volume of the reactants and the products of the reaction are different, then the system will generate local density gradients, which give rise to forces that induce fluid motion in the system. [18][19][20] If the products are denser (occupy less volume) than the reactants and the enzyme-attached patch is located on the top wall of the fluid-filled chamber, then the dense, product-laden fluid at the patch initially moves downward and forms convective rolls that circulate towards the patch on the top (as indicated by the direction of the arrows in Fig. 1C).…”

Open Questions of Chemically Powered Nano- and Micromotors

“…In this case, a micromotor is also a moving pump. , Note that, in addition to the flows generated by the motion itself, a micromotor can significantly change the density of its surrounding liquid as a result of an endo- or exothermic reaction or from the accumulation or consumption of heavy/light molecules. Such buoyancy-induced convective flows have been well documented for catalytic − and electrical micropumps (that are fixed in space), but often wrongly neglected for moving micromotors.…”

Self‐Propelling Macroscale Sheets Powered by Enzyme Pumps

“…[17,18] Earlier analytical models showed that the pumping speed increased with the height of the chamber as h 3 , [8,16] suggesting amplification of the pumping speed in taller chambers. Experimentally, Gao et al [19] recently reported scaling effects in urease pumps and found that larger areas (5 mm in diameter) of the enzyme coating will lead to faster fluid speed in micro-chambers.…”